Multiple polarization spiral antenna

ABSTRACT

APPARATUS AND A METHOD OF EXCITING A MULTI-ELEMENT LOG SPIRAL ANTENNA TO OBTAIN LEFT-HAND AND RIGHT-HAND CIRCULAR POLARIZATION RADIATION. THE ANTENNA ELEMENTS ARE INTERWOUND IN THE USUAL MANNER IN TE FORM OF EITHER EQUIANGULAR OR ARCHIMEDES SPIRALS TERMINATED AT A CIRCUMFERENCE CONSISTENT WITH A DESIRED RADIATION PATTERN. EACH ELEMENT OF THE ANTENNA RECEIVES MULTIPLE CURRENT MODE EXCITATION AT THE INNER TERMINALS. TYPICALLY, AN ANTENNA EXCITED BY FIRST, SECOND, FOURTH, AND FIFTH MODE CURRENTS PRODUCES BOTH LEFT-HAND AND RIGHT-HAND, FIRST AND SECOND MODE, CIRCULAR POLARIZED RADIATION WHEN TERMINATED AT AN APPROPRIATE CIRCUMFERENCE.

Feb. 1971 SAMUEL CHUNG SHU KUO ETA!- 3,562,755

MULTIPLE POLARIZATION STIR/U1 [\N'l'V-NNA Filed June 5, 1968 2 Sheets-Sheet 1 FIG./

FIG. 2A

INVENTOR SAMUEL CHUNG-SHU KUO CHARLES CHUNG-YEH LIU ys/WM aw ATTOR NEY F 1971 SAMUEL CHUNG-$HU KUO EM 3,

MULTIPLE POLARIZATION SPIRAL ANTENNA Filed June s. 1968 2 Sheets-Sheet 2 (f O 0 O 0 T DELAY DELAY DELAY DELAY DELAY LINE LINE LINE LINE LINE 2O 26 3o 3/ 32 33 32 I OSCILLATOR 4 FIG 4 INVENTOR SAMUEL CHUNG-SHU KUO CHARLES CHUNG-YEH LIU S XZW ATTORNEY United States Patent MULTIPLE POLARIZATION SPIRAL ANTENNA Samuel Chung-shn Kuo, San Jose, Calif., and Charles Chung-yeh Lin, Dallas, Tex., assignors to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Filed June 3, 1968, Ser. No. 734,001 Int. Cl. H01q 1/36, 3/26 US. Cl. 343-895 12 Claims ABSTRACT OF THE DISCLOSURE N This invention relates to a spiral antenna and more particularly to an antenna adapted to transmit or receive left and right-hand circular polarization in several modes.

Spiral antennas produce a clean radiation pattern in a relatively compact configuration and are considered to have many advantageous features. They have been described in numerous patents issued within the last several years. Spiral antennas are conventionally fed from the inner terminals of the various interwound elements and the applied currents travel away from the center in the direction of the spiral winding. In some special applications, spiral antennas have been fed from the outer terminals and the currents travel opposite to the winding direction of the antenna elements. In accordance with a standard IEEE definition, the winding direction of a spiral antenna is determined from the center terminals opposite to the winding direction of the antenna elements.

The polarization sense of a planar log spiral antenna may be determined by the Hand Rule wherein the thumb is pointed in the direction of the radiating fields, and the fingers in the direction of the spiral arm currents. Thus, a right-hand wound spiral, that is, an antenna whose elements are wound in a counterclockwise direction, normally produces a right-hand circular polarized field when excited from the inner terminals since the currents travel in a counterclockwise direction. A right-hand wound spiral antenna fed from the outer terminals normally produces left-hand circular polarized fields, again in accordance with the Hand Rule.

Heretofore, to produce both left-hand circular and right-hand circular polarized radiation, it has been the practice to use a single planar log spiral antenna fed from both the inside terminals and outside terminals, simultaneously. There are, however some difficulties encountered when feeding a spiral antenna from the outer terminals; such as, impedance matching, bulk beam forming and processing networks, and a large number of input terminals. Thus, it would be advantageous if a single antenna used for communications, direction finding, tracking or homing, could provide both left-hand and right-hand circular polarized radiation by feeding only the inner terminals of the various antenna elements.

The present invention is directed to a multi-element log spiral antenna radiating circular polarization fields in both the left-hand and the right-hand direction. This 'ice polarization diversity will be obtained by exciting each antenna element with several current modes simultaneously at the inner terminals. The maximum number of current modes possible to excite a given antenna configuration is one less than the total number of antenna elements. Thus, for a six element log spiral antenna there are five current modes which can be connected simultaneously to each element. In accordance with this invention, a spiral antenna with the proper element dimensions can be fed from the inner terminal to obtain either first (2) or second (A) mode radiation, or both simultaneously, in both senses of circular polarization. 7 W

Accordingly, an object of the present invention is to provide a spiral antenna having polarization diversity. Another object of the invention is to provide polarization diversity by multi-mode excitation of each antenna element. A further object of the invention is to provide a spiral antenna having polarization diversity wherein each antenna element is excited at one terminal. An additional object of the invention is to provide a method of exciting a spiral antenna to produce both-right-hand and left-hand circular polarization. Still another object of the invention is to provide a method of exciting a spiral antenna simultaneously with multiple current modes. Yet another object of the present invention is to provide an antenna for generating first or second mode radiation, or both simultaneously, in both senses of circular polarization.

Other objects and advantages will be apparent from the specification and claims and from the accompanying drawings illustrative of the invention.

Referring to the drawings:

FIG. 1 shows schematically an elementary four element equiangular spiral antenna,

FIGS. 2A and 2B illustrate current vectors for inner and outer first mode excitation, respectively, for a four element spiral antenna,

FIG. 3 illustrates schematically a six element right hand wound log spiral antenna with first mode and second mode phase angles at the inner terminals and first mode and second mode phase angles at the outside terminals, and

FIG. 4 is a block diagram of a signal processing system for a polarization diversity antenna in accordance with the present invention.

Although the techniques of beam forming for a log spiral antenna have not been extensively documented, they are comprehended rather thoroughly by those actively engaged in antenna technology. Where the techniques of beam forming for spiral antennas have been discussed, an analogy is often made between the two element spiral antenna and a two Wire transmission line. Such discussions are believed adequate for an understanding of the operation of the basic spiral antenna and easily expanded to understand the operation of a multi-element log spiral antenna. In the process of describing spiral antenna operations, there has been formulated what is known as the Current Band theory which states that maximum radiation occurs at the one, two and three wavelength circumferences when the antenna elements are excited with first, second and third mode currents. However, the Current Band theory cannot be applied in predicting the location of active regions, that is regions of radiation, on any log spiral antenna since the radiation patterns and location vary as the function of the wrap angle (or). For a very tightly wound log spiral antenna, the radiation regions of various current modes can be fairly accurately located by this theory. This invention may best be understood by first considering a four element log spiral antenna such as shown in FIG. 1 which has four spiraling elements l0l3 wound in a counterclockwise direction. As illustrated,

these elements form equiangular spirals which are defined by the equation:

p=po l l where p and are conventional polar coordinates, p the radius of the inner terminal aperture, and a a constant greater than zero. Note, the winding direction of the spirals is determined from the center terminals to the outer terminals when using the above equation. The constant a is determined from the relationship Where a designates the angle formed by a radius vector and a tangent to the curve at a point of intersection with the radius vector. The equiangular spiral curve derives its name from the fact that the angle 0: remains constant at all points on the curve. Thus, the constant a determines the rate of spiral of the antenna element curve.

Consider the radiating system of FIG. 1 wherein a number of identical elements have N-fold symmetry: i.e., a rotation about the antenna axis through an angle equal to (Zr/N) leaves the structure unchanged. As a function of azimuthal angle 4:, the far field characteristics of such an antenna are given by where 111 represents the excitation current modes and has a maximum of three for a four element antenna. The

three ways of exciting a four element antenna are identified as mode one, mode two and mode three as given by the expressions 1=( i, 1, j)mode one 4) 2=( 1,1, *1) mode two 5 3=( J, -1, 1') mode three (6) where A,,: (I I I I is the current vector notation for the excitation of the four input terminals numbered in a clockwise direction around the antenna feed circle. Denoting the input current at the nth terminal by I current mode excitation of each element can be described by the vector n 1 2 IN1) where and N equals the number of antenna elements.

Excitation of the antenna elements 10-13 in mode one with the phase pattern (1, +j, -1, j) produces a field which has an azimuthal variation such that a rotation of 1r/2 radians about the antenna axis in the direction of an increasing p is accompanied by (a) a phase delay or 1r/ 2 radians, (b) a phase avance 31r/ 2 radians,

where right-hand circular polarization, as previously described, corresponds to a phase delay and left-hand circular polarization corresponds to a phase advance in the direction increasing 4 Since a right-hand circular polarized antenna produces field of the form v where m=1, mode one current excitation produces first mode radiation from a right-hand circular polarized antenna on which the-currents are traveling in a counterclockwise direction. Similarly, since a left-hand circular polarized antenna requires a field with azimuthal variation in the form j3 where m=3, mode one excitation is said to yield third mode radiation for a left-hand circular polarized antenna on which the currents travel in a clockwise direction. With reference to the antenna of FIG. 1, mode one excitation of the inner terminals produces counterclockwise currents and mode one excitation of the outer terminals produces clockwise currents. The radiation modes produced by exciting the antenna elements with current modes two and three are apparent from similar analysis. The results are listed in Table 1.

TABLE 1.RELATION BETWEEN SENSE OF POLARIZATION AND MODE OF EXCITATION OF A FOUR-ARM LOG- SPIRAL ANTENNA Right-hand circular polarized spiral antennas Excitation current Left-hand circular polarized spiral antennas vectors Mode 1. Mode 2. Mode 3- Referring now to FIG. 2 there is shown the current vectors for the planar, four-element, right-hand wound, log spiral antenna of FIG. 1 developed by mode one current excitation. FIG. 2A shows the current vector for each element for mode one excitation when the antenna is fed from the inner terminals and FIG. 2B shows the current vectors for each element for mode one excitation of the outside terminals. Mode one excitation of the inside terminals produces right-hand circular polarization and mode one excitation of the outside terminals produce s left-hand circular polarization.

Excitation of properly designed log spiral antenna in the mode m produces a phase delay of 2m1r/ N between each subsequent terminal in the clockwise direction. Radiation takes place in an annular ring approximately enlosed by (m-l))\ c (m+l))\, where 0 equals the annular ring circumference and k the free space wavelength. The current wave travels along each arm with little attenuation until it reaches the radiation region where it decays rather rapidly due to radiation. From experimental data, it has been shown that when a four element antenna, such as illustrated in FIG. 1, is excited with first mode currents, A (I, j, -l, j), the near field amplitude takes a sharp rise from the feed terminals and reaches a maximum at approximately c=0.6 and then decreases rather rapidly and drops 10 db down at c=l.757\. Thus, the deduction that the active region of such an antenna for first mode excitation is an annular ring approximately enclosed by 0.6 c 1.75 When the antenna is excited with second and third mode currents, the active regions are found to be enclosed by approximately 1.2 c 2.9 and 27\ c 2.75 respectively.

As mentioned, the currents travel along the antenna elements with little decay until they reach the active region where they are greatly attenuated due to radiation. If the antenna has a circumference of less than 2%, then from the previous discussion it can be shown that the third mode currents travel along each element with little attenuation and upon reaching the outer terminations are reflected therefrom. The reflected currents maintain the same relative current vector A =(i, j, -l, 1'), but now travel in a direction opposite from the initial excitation direction. Third mode reflected currents continue to travel through the elements with little attenuation until they reach the radiation region within the limits of 2 c 2.75 from the inside terminals, or until they reach a region of first mode radiation for first mode currents connected to the outside terminals.

Referring to FIG. 2B, reflected third mode excitation resembles first mode excitation of the outer terminals as described previously. Assume the first mode active regions of the antenna of FIG. 1, either excited from the inside or the outside terminals, are located approximately at the same region, then the antenna will furnish first mode radiations of circular polarization in opposite senses simultaneously. Thus, when a right-hand wound, four element, log spiral antenna has a circumference of less than 2x at P the lowest bandwith frequency, the antenna will furnish first mode radiation of right-hand circular polarization and first mode radiation of left-hand circular polarization when fed from the inside terminals by first and third mode current excitation.

Since there are only N1 fundamental ways to excite an N-element spiral antenna, to obtain dual mode opera- TABLE 2 CURRENT VECTOR AT INNER TERMINALS A 60, 120, 180, 240, 300 Mode 1 A2=(O, 120, 240, 0, 120, 240 Mode 2 A,= 0, 180, 0, 180, 0, 180 Mode 3 A,= 0, 240, 120, 0, 240, 120, Mode 4 A5=(0, 300, 240, 180, 120, 60) Mode 5 If the same antenna is fed from the outer terminals, the five possible ways of excitation (in a clockwise direction) are TABLE 3 CURRENT VECTORS AT OUTER TERMINALS A1=(O, 300, 240, 180,120, 60 m=1 A2=(0, 240, 120, 0, 240, 120 m=2 A,=(0, 180, 0, 180, 0, 180) m=3 A,= 0, 120, 240, 0, 120, 240 m=4 A5=(0, 60, 120, 180, 240, 300 m==5 where the above current vectors are calculated in accordance with the discussion of FIG. 1.

Again by an extension of the discussion with regards to the four element antenna of FIG. 1, the approximate active regions of the various current modes can be predicted and are tabulated below:

TABLE 4 [Approximate location of active regions of various modes of a six-arm, a=80, planar log-spiral] Approximate location of Mode of excitation active region m= 0.6)\ c 1.75)- m=2 1.25)\ c 2.9 m=3 2)\ e 3.75)\ m= 2.75)\ c 4.6)\ m=5 3.5)\ c 6.6).

Thus, if a fairly tight, right-hand wound, six element log spiral antenna has a circumference on the order of 2.75 wavelengths at F there would be first and second mode radiation produced. For the antenna of FIG. 3, right-hand circular polarization for first and second mode radiation will be produced when currents are applied to the inner terminals. If the inner terminals are also fed fourth mode and fifth mode currents, the circumference of the antenna is insufficient to support fourth and fifth mode radiation fields. Currents for these two modes will travel on the spiral elements with negligible attenuation until reaching the outer terminals where they will be reflected. The current vectors of the reflected currents are facsimiles of the current vectors of the first and second mode currents applied to the outer terminals as given in the Table 3. FIG. 3 shows the current vectors at each input terminal for mode four and five excitation of the inside terminals and mode one and two excitation of the outside terminals. Taking the fourth mode excitation for example, the current wave will travel along each element with little attenuation until it reaches the radiation region, which is located approximately in the annular ring enclosed by 3)\ c 5 Where it will decay due to radiation. However, if the antenna is truncated at a 2.75 wavelength circumference, the currents will be reflected back by the truncation as an outside feed and travel as second mode radiation approximately located in an annular ring enclosed by c 3)\ in the left-hand sense from the outer terminals. Second mode radiation of left-hand circular polarization will take place since the relative phasing of the reflected currents with respect to each other are unchanged and coincide with the second mode excitation of a left-hand wound spiral antenna.

Similarly, first mode radiation of left-hand circular polarization can be obtained by exciting an anetnna truncated at approximately 2.75 wavelengths with fifth mode currents at the inner terminals. Hence, the antenna of FIG. 3 will radiate right-hand circular polarized fields in the first and second mode by exciting the inner terminals with first and second mode currents, and left-hand circular polarized fields will be obtained by exciting the inner terminals with fourth and fifth mode currents.

A simple center fed log spiral antennaradiating both first and second mode, left-hand and right-hand circular polarization, can be obtained by feeding an N-arm (N being a minimum of 6) log spiral antenna in the first, second, N-l and N2 current excitation modes. By increasing the number of spiral elements, more separation will be introduced between the active regions of the second and N2 rnode radiation, thus increasing the bandwidth of the system. An octave bandwidth can be achieved by using an eight element spiral antenna.

There are any number of systems available to generate the proper phase shifts between the currents applied to the antenna elements. One of the simplest devices of obtaining a phase shift between currents coupled to the antenna elements is a length of transmission line. A transmission line phase shifter falls within a fixed phase shift classification; other phase shift classes include variable phase shifters actuated by mechanical means, and variable phase shifters controlled by electronic means.

Referring to FIG. 4, there is shown a simplified schematic of a phase shifting system for generating one of the current modes to a transmitting antenna. An oscillator 20 generates a signal at frequency F on a transmission line 22. This signal passes through a delay line 24 before coupling to one of six elements of the antenna as shown in FIG. 3. The output of the oscillator 20 also passes through a phase shift network 26 the output of which has been phase shifted with respect to the oscillator output. For a right-hand wound, log spiral antenna having six elements, the phase angle between the input and output of the phase shift network 26 equals 60 for first mode current. The output of the phase shift network 26 passes through a delay 28 before coupling to a second element of the antenna shown in FIG. 3. The system of FIG. 4 is repeated for each of the six elements of the antenna and includes phase shift networks 30-33 and delay lines 34-36. For first mode currents coupled to a right-hand wound spiral antenna, each of the phase shift networks 30-33 increases the phase angle by 60 over its input signal. By properly inserting delay lines before each element the currents are fed to the elements simultaneously. Thus, each of the delay lines 24, 28, and 34 through 36 must be adjusted depending on its position within the system.

To feed the antenna of FIG. 3 with four current modes requires the system of FIG. 4 to be repeated for each of the various modes. For mode two and mode four excitation, the phase shift networks introduce a phase delay to their input signals. Note, the coupling arrangement of the outputs of the phase shift networks to the antenna elements varies between mode two and mode four excitation to produce the proper phase relationship between the antenna elements. For mode three excitation the phase shift networks would introduce of phase delay, and for mode five excitation 60 of phase delay.

While several embodiments of the invention, together with modifications thereof, have been described in detail herein and shown in the accompanying drawings, it will be evident that various further modifications are possible without departing from the scope of the invention.

What is claimed is:

1. A spiral antenna comprising:

a plurality of antenna elements wound about each other and starting at a central aperture,

means for generating a maximum of N1 current modes for energizing each antenna element simultaneously, where N equals the total number of antenna elements, and

means for coupling all of said generated current modes to said central aperture of said antenna to generate simultaneously left and right-hand circular polarizaw tion. 2. A spiral antenna as set forth in claim 1 wherein said generating means produces current modes for energizing adjoining antenna elements (21rm/N) out of phase, where m identifies the mode.

3. A spiral antenna as set forth in claim 1 wherein antenna elements are Archemedian spirals.

4. A spiral antenna as set forth in claim 3 wherein said antenna elements are logarithmic spirals.

5. A spiral antenna for obtaining polarization diversity comprising:

at least six spiral antenna elements wound about each other and having a central aperture feed point,

means for generating at least five current modes for each of said six antenna elements, each current mode energizing adjacent elements with currents (21rm/N) out of phase, where m identifies the mode, and N the number of spiral elements, and

means for coupling said current modes to the individual central aperture feed points of said antenna elements to obtain opposite sense circular polarizations simultaneously.

6. A spiral antenna for obtaining polarization diversity as set forth in claim 5 wherein said antenna elements have a circumference on the order of 2.75 wavelengths.

7. A spiral antenna for obtaining polarization diversity as set forth in claim 6 wherein said generating means produces first, second, fourth and fifth mode currents.

8. A spirit antenna for obtaining polarization diversity as set forth in claim 5 wherein said first and second current modes generate one sense circular polarization and said fourth and fifth current modes generate the opposite sense circular polarization radiation.

9. A method of obtaining polarization diversity from a spiral antenna having N antenna elements comprising: generating up to N 1 current modes for each antenna element, each current mode having N excitation currents out of phase with respect to each other, and

energizing each of said antenna elements simultaneously at a central terminal aperture with all of said generated current modes to produce opposite sense circular polarizations simultaneously.

10. A method of obtaining polarization diversity as set forth in claim 9 wherein the first, second, fourth and fifth current modes are generated for an antenna having six antenna elements.

11. A method of obtaining polarization diversity as set forth in claim 10 wherein said fourth and fifth current modes are reflected from the outer terminations of said antenna elements to produce circular polarization in the opposite sense from said first and second modes.

12. A method of obtaining polarization diversity as set forth in claim 9 wherein the excitation currents are (21rm/N) out of phase with respect to each other, where m identifies the current mode.

References Cited UNITED STATES PATENTS 3,137,002 6/1964 Kaiser et a1. 343895X 3,144,648 8/1964 Dollinger 343-895X 3,344,425 9/1967 Webb 343.895X 3,373,433 3/1968 Blaisdell 343895X HERMAN K. SAALBACH, Primary Examiner T. VEZEAU, Assistant Examiner U.S. Cl. X.R. 

